We had a debate in our postnatal class this week as to whether or not breast milk is adversely affected by exercise.

My belief is that moderate exercise does not have an impact on your breastmilk, nor your baby's acceptance of it.
Exercise is in fact a positive element regarding breastfeeding, and many women find that their milk flows more freely with exercise as well as good hydration habits (drinking of water).

Extreme athletic forms of exercise may cause a lactic acid build up which my discourage your baby from feeding for an hour after this excessive training session. In this case, I recommend that you could express milk to bottle feed to your baby so that any lactic acid is not present if you baby seems offended by the post exercise milk you produce.

For some more formal information follow these links:

http://www.nursingbaby.com/nursing/exerevi.htm

http://pediatrics.aappublications.org/cgi/content/abstract/109/4/585